The new coronavirus has infected nearly 90,000 people, and more than 3,000 have died. But relatively few children appear to have developed severe symptoms so far, according to available data.

“Disease in children appears to be relatively rare and mild,” with those under 19 years making up only 2.4 percent of the total cases, according to a report published Feb. 28 by the World Health Organization.

So why aren’t more children getting sick?

“My strong, educated guess is that younger people are getting infected, but they get the relatively milder disease,” Dr. Malik Peiris, chief of virology at the University of Hong Kong, said last month. Dr. Peiris has developed a diagnostic test for the new coronavirus.

The numbers so far support that theory: According to the W.H.O., only 2.5 percent of those under 19 have developed severe disease and only 0.2 percent had critical disease. There have been no deaths recorded in children under 9.

Scientists may not be seeing more infected children because “we don’t have data on the milder cases,” Dr. Peiris said.

Without more information, it is also unclear whether children can transmit the disease to others. The W.H.O. report said that its team sent to China, the epicenter of the outbreak, “could not recall episodes in which transmission occurred from a child to an adult.”

Still, children who are detected as infected must be shedding some virus or they wouldn’t be detected, noted Dr. Marc Lipsitch, an epidemiologist at the Harvard T.H. Chan School of Public Health. But whether their infectiousness is high is as yet unknown. “It’s a very high priority to do studies to find it out,” he said.

One way to find out, he said, is to look at outbreaks such as the one at the church in South Korea. “If there were children among those people,” he said, “that would be a goldmine of data.”

The other approach is to conduct household studies, where multiple members of a family might be infected.

The child, too, had signs of viral pneumonia in the lungs, doctors found — but no outward symptoms. Some scientists suspect that this is typical of coronavirus infection in children.

“It’s certainly true that children can be either asymptomatically infected or have very mild infection,” Dr. Raina MacIntyre said last month. Dr. MacIntyre is an epidemiologist at the University of New South Wales in Sydney, Australia, who has been studying the spread of the new coronavirus.

In many ways, this pattern parallels that seen during outbreaks of SARS and MERS, also coronaviruses. The MERS epidemics in Saudi Arabia in 2012 and in South Korea in 2015 together claimed more than 800 lives. Most children who were infected never developed symptoms.

No children died during the SARS epidemic in 2003, and the majority of the 800 deaths in the outbreak were in people over age 45, with men more at risk.

Among the more than 8,000 cases of SARS, researchers at the Centers for Disease Control and Prevention were able to identify 135 infected children in published reports.

Children under age 12 were much less likely to be admitted to a hospital or to need oxygen or other treatment, the researchers found. Children over age 12 had symptoms much like those of adults.

“We don’t fully understand the reason for this age-related increase of severity,” Dr. Peiris said. “But we see that now — and with SARS, you could see that much more clearly.”

It’s not unusual for viruses to trigger only mild infections in children and much more severe illnesses in adults. Chickenpox, for example, can be largely inconsequential in children, yet catastrophic in adults.

Influenza is unusual in that it has evolved with humans over thousands of years and infects millions worldwide each year. Still, even though thousands of young children end up in the hospital each year with influenza, just a small percentage of them die, said Dr. Krys Johnson, an epidemiologist at Temple University in Philadelphia.

This trend is generally true of respiratory illnesses because children tend to eat well, and to get plenty of exercise and rest — none of which may be true of adults. “The younger, most healthy segment of the population are able to fight it off,” she said. Adults may also be more susceptible because they are more likely to have other diseases, such as diabetes, high blood pressure or heart disease, that weaken their ability to stave off infections.

Frequently Asked Questions and Advice

Updated June 5, 2020

How many people have lost their jobs due to coronavirus in the U.S.?

The unemployment rate fell to 13.3 percent in May, the Labor Department said on June 5, an unexpected improvement in the nation’s job market as hiring rebounded faster than economists expected. Economists had forecast the unemployment rate to increase to as much as 20 percent, after it hit 14.7 percent in April, which was the highest since the government began keeping official statistics after World War II. But the unemployment rate dipped instead, with employers adding 2.5 million jobs, after more than 20 million jobs were lost in April.

Will protests set off a second viral wave of coronavirus?

Mass protests against police brutality that have brought thousands of people onto the streets in cities across America are raising the specter of new coronavirus outbreaks, prompting political leaders, physicians and public health experts to warn that the crowds could cause a surge in cases. While many political leaders affirmed the right of protesters to express themselves, they urged the demonstrators to wear face masks and maintain social distancing, both to protect themselves and to prevent further community spread of the virus. Some infectious disease experts were reassured by the fact that the protests were held outdoors, saying the open air settings could mitigate the risk of transmission.

How do we start exercising again without hurting ourselves after months of lockdown?

Exercise researchers and physicians have some blunt advice for those of us aiming to return to regular exercise now: Start slowly and then rev up your workouts, also slowly. American adults tended to be about 12 percent less active after the stay-at-home mandates began in March than they were in January. But there are steps you can take to ease your way back into regular exercise safely. First, “start at no more than 50 percent of the exercise you were doing before Covid,” says Dr. Monica Rho, the chief of musculoskeletal medicine at the Shirley Ryan AbilityLab in Chicago. Thread in some preparatory squats, too, she advises. “When you haven’t been exercising, you lose muscle mass.” Expect some muscle twinges after these preliminary, post-lockdown sessions, especially a day or two later. But sudden or increasing pain during exercise is a clarion call to stop and return home.

My state is reopening. Is it safe to go out?

States are reopening bit by bit. This means that more public spaces are available for use and more and more businesses are being allowed to open again. The federal government is largely leaving the decision up to states, and some state leaders are leaving the decision up to local authorities. Even if you aren’t being told to stay at home, it’s still a good idea to limit trips outside and your interaction with other people.

What’s the risk of catching coronavirus from a surface?

Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

How can I protect myself while flying?

If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

Should I wear a mask?

The C.D.C. has recommended that all Americans wear cloth masks if they go out in public. This is a shift in federal guidance reflecting new concerns that the coronavirus is being spread by infected people who have no symptoms. Until now, the C.D.C., like the W.H.O., has advised that ordinary people don’t need to wear masks unless they are sick and coughing. Part of the reason was to preserve medical-grade masks for health care workers who desperately need them at a time when they are in continuously short supply. Masks don’t replace hand washing and social distancing.

What should I do if I feel sick?

If you’ve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

The body’s innate immunity, which is critical for fighting viruses, also deteriorates with age, and particularly after middle age.

“Something happens at age 50,” Dr. MacIntyre said. “It declines, and it declines exponentially, which is why for most infections we see the highest incidence in the elderly.”

A key question about the new coronavirus is whether children who are infected and asymptomatic are able to pass the virus to others.

“We know that young people in general — not just kids, but young adults and teenagers — have the most intense contact in society,” Dr. MacIntyre said. Young people who don’t realize they are sick may contribute to the epidemic’s momentum, she said.

To understand the epidemic fully, she and other scientists said they need detailed data: when people were first exposed to the virus, when they first began to show symptoms, how many and which people have mild symptoms versus more severe disease.

With detailed data, some observations, such as the higher risk in men, may change. Still, Dr. Mark Denison, a pediatric infectious diseases specialist at Vanderbilt University in Nashville, said last month that he does not expect to see a sudden uptick in infected children.

“It’s hard for me to imagine that there’s such a degree of underreporting of clinical illness in children that we’re only hearing about two or three cases,” he said.

“I think it means that there are many, many less children” who are infected in China, he said, “and that they’re not as much at risk.”